U.S. Pat. No. 8,093,758 describes a power transfer system for inductively coupled power transfer (ICPT). It describes a circuit having a primary conductive path supplied with alternating current from a power supply comprising a resonant converter. It also describes inductively coupling a secondary circuit to the first circuit using a transformer. The secondary circuit comprises a winding and a capacitor whereby the capacitor is connected in parallel to the winding. The winding forms an inductive element. A load is parallel to the inductive element and the capacitor. A controlled shorting switch is used to decouple the pick-up and thereby regulate the impedance to the primary conductive path. Without the short, the secondary circuit would resonate. The controlled shorting switch is parallel to the inductive capacitor and the inductive element. Because the controlled shorting switch causes large conduction losses and to allow for frequency variations of the current in the primary path, U.S. Pat. No. 8,093,758 describes the use of a variable inductor or a variable capacitor in parallel to the inductive element and the capacitor. The power transfer system comprises sensing means to sense a condition of the load and control means to selectively tune or de-tune the secondary circuit in response to load conditions sensed by the sensing means by varying the effective capacitance or inductance of the secondary circuit to control the transfer of power to the secondary circuit dependent on the sensed load condition. The control means comprises two switches with appropriate drivers to control the current flowing through an inductor or capacitor that thereby becomes variable. The sensing means sense the phase of a voltage in the resonant circuit. The control means is adapted to drive switching means to connect the variable inductor or to disconnect the variable capacitor to the secondary circuit a predetermined time period after a voltage zero crossing. Thereby the control means varies the variable inductance or variable capacitor such that the resonant frequency is detuned away from resonance (the track frequency) when the load is small and does not need high power to be transferred through the secondary circuit. The secondary circuit is tuned toward the track frequency as the load increases and high power transfer is required to satisfy the demand from the increased load.
A disadvantage of the power transfer system as described in U.S. Pat. No. 8,093,758 is that a relatively expensive controller is needed and that the output voltage to the load must be sensed. Moreover, the switches cause noise in the pick-up circuit and therefore on the primary path when switching. This noise may disturb the function of other pick-ups coupled to the cable or may require that the power supply is robust against this noise in that the alternating current is not effected and remains as intended.